Sintering is a solid state process that transforms compacted mechanical bonds between powder particles into solid metallurgical bonds between the particles when heated. Sintering is a widely used die-to-substrate joining technique which allows for an increased lifetime and an enhanced power density of power modules. The die-substrate joint formed by sintering is based on metal inter-diffusion at an enhanced surface area of micrometer or nanometer-sized particles. These metal particles are typically compressed at high temperatures, with or without applied pressure. Common sintering pastes used for joining dies to substrates consist of silver particles coated by organic complexes and organic ingredients such as solvents to hold the silver particles together.
One conventional sinter die attachment technique involves firmly placing the dies onto a substrate with a wet silver paste coating. The organic components of the wet silver paste provide sufficient adhesion for holding the dies to the substrate. However, during the subsequent sintering process, drying channels are formed as the organic composites evaporate. These channels severely degrade the thermal and electric properties of the sintered silver layer. Consequently, the reliability requirements, in particular the module life time, cannot be fulfilled.
Most of the solvent can be removed from the silver paste prior to die placement on the substrate by drying the silver paste at an elevated temperature. However, removing most of the solvent from the sintering paste prior to die placement on the substrate renders it difficult to hold the dies in position on the substrate, because the sintering material has poor adhesion qualities once dried. The dies must remain in proper position on the substrate after the placement process and during transport of the substrate to the sintering equipment, which proves difficult when using dried sintering pastes.